1. Field of the Invention
The present invention relates to a stereoscopic retinal camera and, more particularly, to a focus detecting system to be incorporated into a stereoscopic retinal camera.
2. Description of the Prior Art
In focusing a conventional stereoscopic retinal camera on the spatial image of the retina, the spatial image is superposed on the reticule of a viewing system and the focusing condition of the spatial image is observed through the viewing system. Since the condition of the image in the viewfinder is directly dependent on the refracting power of the eyes of the observer, the refracting power of the viewfinder must be exactly adjusted and accurate focusing requires highly skillful focusing technique.
Referring to FIG. 10, a monocular retinal camera disclosed in Japanese Patent Publication (Kokoku) No. Sho 61-39050 comprises: a photographing optical system comprising an objective lens 51 to be disposed opposite to the eye E, a diaphragm 52 in a conjugate relation with the pupil EP of the eye E, a focusing lens 53, an image-forming lens 54 and a film 55 (the focusing lens 53 and the image-forming lens 54 constitute an afocal optical system); a viewfinder optical system comprising a reflecting mirror 72 disposed before the film 55, a field lens 73 disposed on a path along which a light beam reflected by the reflecting mirror 72 passes, a reflecting mirror 74 and an ocular 75; an illuminating optical system comprising a perforated reflecting mirror 56 disposed at a position before the diaphragm 52 on the light path of the photographing optical system, a relay lens 57 disposed on a path along which a light beam reflected by the perforated reflecting mirror 56 passes, a condenser lens 59, a circular slit plate 60, a flash lamp 61 for stroboscopic photography, a heat filter 62, a condenser lens 63 and an illuminating light source 64; and an index projecting system for facilitating focusing, comprising a reflecting mirror 65 disposed at a position after the diaphragm 52 on the light path of the photographing optical system, a reflecting mirror 66, a relay lens 67, a slit index 68, a deflecting prism 69 disposed contiguously with the slit index 68, a condenser lens 70 and a light source 71.
Light beam emitted by the illuminating light source 64 is reflected by the perforated mirror 56 and the reflected light beam passes through the objective lens 51 to illuminate the fundus ER. The light path of the index projecting system is branched from the light path of the photographing optical system by the reflecting mirror 65, i.e., a light path dividing means, disposed behind the perforated mirror 56. Thus, the focusing operation is not effected by illuminating condition, and the index projecting system and the photographing optical system are interlocked simply. Since the index illuminating light beam passes through the space between the illuminating light beam and the photographing light beam or overlap part of the illuminating light beam, the path of the index illuminating light beam needs delicate positional adjustment. The light path dividing means must be positioned so that the light path dividing means may not intercept the light beam of the focusing lens system.
A monocular retinal camera disclosed in Japanese Patent Publication (Kokoku) No. Sho 57-12294 is provided with a direct-vision viewfinder to observe the image of an index formed on the fundus by projecting a plurality of light beams cooperatively forming the image of the index for focusing, i.e., focus detection.
As shown in FIG. 11, the monocular retinal camera has a photographing optical system and an illuminating optical system comprising an objective lens 81, a photographing lens 82 serving also as a relay lens, a film 83, a diaphragm 84, a perforated mirror 85 for reflecting illuminating light, provided with a central hole, an illuminating light source 86, a condenser lens 87, a relay lens 88, and a ring slit 89. A light beam emitted by the illuminating light source 86 is converged on a focal point F1 by the condenser lens 87, gathered on the ring slit 89 by the relay lens 88, reflected by the perforated mirror 85 toward the objective lens 81 and focused on the pupil EP of the eye E by the objective lens 81 to illuminate the fundus EF. The image light reflected by the fundus EF passes through the refraction system of the eye E and the objective lens 81, is focused on the rear focal point F2 of the objective lens 81, passes through the hole of the mirror 85 and the diaphragm 84, and focused on the film 83 by the photographing lens 82 to form the image of the fundus EF on the film 83. The relay lens 88 of the illuminating optical system is equivalent to the photographing lens 82 of the photographing optical system. The relay lens 88 and the photographing lens 82 are equidistant from the perforated mirror 85. A half mirror 90 is disposed on the light path of the illuminating optical system before the relay lens 88, i.e., at a position on the side of the light source 86 with respect to the relay lens 88, so that the image of the index projected by an index projecting system is reflected toward the respective light paths of the illuminating optical system and the photographing optical system so that the image of an index is projected on the fundus EF. The index projecting system comprises a light source 91, a condenser lens 92, a slit index 93 disposed near a position where the condenser lens 92 gathers the light emitted by the light source 91, a split image prism 94 disposed near the slit index 93, a relay lens 95, and a two-hole diaphragm 96. The image of the slit index 93 is formed by the relay lens 95 on a plane FJ, which is in a conjugate relation with the film 83. The position of the two-hole diaphragm 96 corresponds to the position of the image of the ring slit 89 formed by the relay lens 88, the distance between the two holes is equal to the diameter of the image of the ring of the ring slit 89. A transparent glass plate 98 having a shading portion 97 of a size large enough to cover the image of the index formed on the fundus EF and capable of being removably inserted in the light path of the illuminating optical system is disposed at a position in a conjugate relation with the plane F3 to prevent the image of the index formed on the fundus EF being obscured by the illuminating light.
A method of focusing the index projecting system will be described hereinafter. The light image of the index 93 is split into two light images by the split image prism 94. Then, the two light images pass through the two holes of the two-hole diaphragm 96, the lenses 88 and 81, and reach the fundus EF to form the images of the index 93 on the fundus EF. The images formed on the fundus EF is observed through a viewfinder optical system comprising a viewfinder mirror 99 capable of being removably placed in the light path between the photographing lens 82 and the film 83, a reflecting mirror 100 and an ocular 101. Then, the photographing lens 82 and the relay lens 88 are shifted so that the film 83 and the plane F3 are in a conjugate relation with respect to the light reflected by the fundus EF, or the film 83, and the index projecting system including the plane F3 are moved to focus the photographing optical system. However, when this index projecting system is incorporated into a stereoscopic retinal camera, the two images of the index are further split if the convergence of the eyes of the observer is not appropriate, which makes focusing difficult, because two images are observed by the stereoscopic retinal camera. Although it may be possible to observe the fundus stereoscopically for minute inspection by the stereoscopic retinal camera provided with a direct-vision viewfinder, the image of the fundus and that of the index overlap each other and part of the fundus cannot be observed.